Deuteronomy 19:1-3, 9

“When the LORD your God cuts off (destroys) the nations whose land He is giving you, and you dispossess them and live in their cities and in their houses, you shall designate three cities for yourself in the central area of the land, which the LORD your God is giving you to possess. You shall prepare and maintain for yourself the roads [to these cities], and divide the territory of your land into three parts, so that anyone who kills another unintentionally may escape there [for asylum].

if you keep and carefully observe all these commandments which I am commanding you today, to love the LORD your God, and to walk [that is, to live each and every day] always in His ways—then you shall add three more cities [of refuge] for yourself, besides these three